This invention relates to active clearance controls for a turbofan engine and particularly to the spray-bar configuration.
U.S. Pat. No. 4,019,320 granted on Apr. 26, 1977 to I. H. Redinger, D. Sadowsky and P. S. Stripinis, and assigned to the same assignee as this patent application discloses and claims spray bars that are externally and circumferentially mounted around the casing of the engine. Air bled from the fan through a manifold feeds these spray bars which in turn judiciously squirts air on the case to control its expansion and contraction. The purpose being is to position the outer air seals relative to the tips of the rotating engine machinery so as to control the gap therebetween. Obviously, the gap should be maintained at a minimum at all modes of engine operation for the entire flight envelope since the gap is a leakage path that adversely affects the efficiency of the rotating machinery, which in turn is reflected in loss of fuel economy. Of course, it is abundantly important to achieve optimum thrust specific fuel consumption. This patent, supra, discloses a spray bar (that is circular in cross section) with discretely located air holes. To satisfy certain ACC applications, it becomes necessary to increase the diameter of the tube which necessitates the tube to be spaced further from the engine case with a consequential loss in cooling effectiveness.
Further the circular cross section does not lend itself to create an optimum film of cooling air between it and the case and hence doesn't take full advantage of the further cooling obtainable from the spent air from the cooling jets that would otherwise scrub the case.
These square pipes not only allow for the more effective distribution of cooling air but also provide a more compact assembly while increasing cooling flow capacity. Thus it is contemplated by this invention that the cooling air is utilized as effectively and efficiently as possible, thereby only utilizing the amount of cooling air necessary to accomplish optimum ACC. This assures that the energy extracted from the engine for ACC purposes does not unduly penalize engine performances which is a consequence of extracting cooling air. Amongst the advantages afforded by the employment of this invention, but not limited thereto are:
(1) the distance between the pipe and the engine case being controlled say, the turbine case, is optimized which places the pipe at 6 to 10 hole diameters from the surface to be cooled; PA1 (2) the spent air lies closer to the engine case surface which scrubs the case which results in better cooling; PA1 (3) cooling air holes can be drilled at locations that allow the cooling jets to strike more responsive points of the engine case; and PA1 (4) the overall spray bar configuration is more compact for the amount of flow capacity they encompass because they follow the engine case contour more effectively.